That's one possibility; but if it stays on for a discernible amount of time, I'd be more likely to suspect high-speed sync or autofocus assist beam. (If it's the latter, there should be a way to set the beam to IR, rather than visible light.)

Re: 600 EX RT firing a blinding light

My 600 EX RT, mounted on a 5d Mark II, has recently started to fire (or rather light up) full power on its own. It stays on like that for a few seconds.

I vaguely remember I had a similar problem with my 430 Canon flash (mounted on the same camera) and that a trainer at a photo workshop helped me solve it by changing the camera settings.

Would someone be able to tell me what I'd need to do with this issue, which is obviously very unpleasant for anyone into whose eyes the flash shines?

Many thanks in advance!

There are two possibilities why you are observing extended lighting up - none will happen on its own, though.

1. You've accidentally activated the modeling flash mode. This happens when you depress the depth of field button which is below the lens release button when the flash is on and mounted. You can disable this by setting the Cfn option in the flash. I would change this to option 1.

2. You've set the flash to MULTI (strobe mode). Change it to ETTL will solve this problem.